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Boosting problems


3000-GTO
12-01-2009, 08:08 PM
My car has not been building boost properly. I have 15g turbos paired with a fmic on a recently rebuilt motor so I know I should be able to do 20psi pretty easily. I have noticed in the morning I can do a good pull in 4th gear and it will easily hit 20psi no problem. However, if I were to drive the car later in the day I'll be luck to hit 16psi. By the way, I have my MBC set for max psi so I know it should be well over 15-16. It confuses me that it consistently will boost that much higher in the morning as opposed to mid-day. I was thinking it might be a cracked manifold or maybe a wastegate stuck open. I am a little skeptical on this theory though beecause why would the car reach a higher psi in the morning when I start driving and not later? If the manifold is cracked wouldn't it always be seeing low boost consistently?

I also tried unhooking both waste gates to see if this made any difference and it boosts exactly the same as if the MBC were still hooked up. I have pressure tested several times and fixed pretty much all the leaks. The only place that air will leak out of is my MBC which is supposed to be normal as far as I know. I was having trouble getting the rear turbo intercooler pipe to seal and bought a new O-ring and it seems better now though. I have datalogged the car and was seeing next to no knock so I don't believe that is an issue either. So I am really lost, if anyone has any ideas please share. Thanks for the help.

EDIT: One thing I forgot to mention, when I go WOT for instance in 2nd gear it will start boosting hard up to 15lbs, but then fall flat on its face and drop to around 10lbs. I know 15g's have got to be able to hold more than 10lbs to redline.

cHo
12-02-2009, 12:00 AM
How are you looking for boost leaks? You should run a little more than what you were planning on boosting on the compressor. Also check your couplers.. Sometimes they'll develop cracks and holes. If you remove the mbc and run the lines from the wastegates straight to the Y pipe, you still boost 15-16psi? You should only be boosting wastegate pressure. And the boosting higher in the morning could be because of the colder denser air in the morning as opposed to the warmer air from mid day. Also do boost tests from 3rd as 1st and 2nd are too short of a gear to be maxing out your boost. And what fuel mods do you have? You don't have any listed and that would be very dangerous if you're doing so on stock fuel.

3000-GTO
12-02-2009, 12:48 AM
How are you looking for boost leaks? You should run a little more than what you were planning on boosting on the compressor. Also check your couplers.. Sometimes they'll develop cracks and holes. If you remove the mbc and run the lines from the wastegates straight to the Y pipe, you still boost 15-16psi? You should only be boosting wastegate pressure. And the boosting higher in the morning could be because of the colder denser air in the morning as opposed to the warmer air from mid day. Also do boost tests from 3rd as 1st and 2nd are too short of a gear to be maxing out your boost. And what fuel mods do you have? You don't have any listed and that would be very dangerous if you're doing so on stock fuel.
Yeah sorry I guess I should have told you more specifically what mods I have done. I have 550cc injectors paired with a TRE fuel pump. I also recently installed a FMIC on the car so the couplers are all new and of course I have no y-pipe anymore either. I agree that I should be boosting more in the morning, but 5psi seems excessive. Also like I mentioned it is running on 15gs so they should boost 20psi even during summer heat. I ran it on the freeway in 4th and was able to reach about 20psi. However, on the way back from class tonight it only got up to 16psi and then drops off.

EDIT: Forgot to mention I look for boost leaks by just listening for leaking air. I might try the soap and water technique too just to make sure I didn't overlook something somehow though.

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